1. Field of the Invention
The present invention relates to a position detecting apparatus, and more particularly, it relates to an alignment apparatus for photoelectrically detecting positions of alignment marks on a substrate such as a semiconductor element or a glass plate through photosensitive material such as photoresist in an exposure apparatus used in the manufacture of semiconductor elements or liquid crystal display elements through a lithographic technique.
2. Description of the Related Art
In exposure apparatuses such as steppers or step-and-scan projection exposure apparatuses, there is provided an alignment apparatus for performing alignment between a reticle (or a photomask and the like) on which a pattern to be transferred is formed and a wafer (or a glass plate and the like) on which photoresist is coated with high accuracy. In order to perform such alignment with high accuracy, it is necessary to correctly detect positions of wafer marks on the wafer.
In this regard, since roughness of a surface of the wafer is changed after an exposure operation and subsequent processes and since heights of the wafer marks in respective layers on the wafer often differ from a height of a background, it is difficult to correctly detect the positions of all of the wafer marks by using a single alignment system. Accordingly, in the past, the following alignment systems have been used for various purposes:
(1) Alignment system of LSA (Laser-Step-Alignment) type:
In this alignment system, a laser beam is directed onto a wafer mark, and light diffracted and scattered from the wafer mark is used to measure a position of such a wafer mark. This alignment system has been widely used regarding a wafer which is being processed.
(2) FIA (Field Image Alignment) system:
This alignment system is a sensor in which an enlarged image of a wafer mark obtained by directing light having a wide wavelength band (emitted from a light source such as a halogen lamp and the like) onto the wafer mark is picked up by an imaging element (such as a vidicon tube or a CCD), and an obtained image signal is image-treated to measure a position of the wafer mark. This system is particularly useful for the measurement of asymmetrical wafer marks on an aluminium layer or a wafer surface. This FIA system is disclosed in co-assigned patent application U.S. Ser. No. 841,833 filed on Feb. 26, 1992, for example.
(3) LIA (Laser Interferometric Alignment) system:
This alignment system is a sensor in which two laser beams having slightly different frequencies are directed onto a diffraction grating wafer mark from two directions, and two diffraction light beams generated are interfered with each other to generate interference light. From the phase of this light position information of the wafer mark is detected. This LIA system is effective for wafer marks having a small height difference or wafer marks having rough surfaces. The principle of detection in the LIA system is disclosed in U.S. Pat. No. 4,710,026 and co-assigned patent application U.S. Ser. No. 418,260 (filed on Oct. 6, 1989), for example.
In the past, these various alignment systems have been used properly in accordance with their purposes. Regarding wafers used in the exposure apparatuses, since a photoresist layer having a thickness of about 0.5-2 .mu.m is normally formed on the entire surface of the wafer, if monochromatic light is used as alignment illumination light or laser beam, interference fringes are generated due to the monochromatic light, thereby causing a detection error when the position of the wafer mark is detected. In order to suppress the interference phenomenon, alignment illumination light of multi-wavelengths has been used or a band of the alignment illumination light has been widened.
For example, in the FIA systems of image pick-up type, a halogen lamp is used as an illumination light source. When a width of a wavelength band (except for a photosensitive band to the photoresist) of the illumination light is selected to be about 300 nm, the light reflected from the surface of the photoresist layer does not substantially interfere with the light reflected from the surface of the wafer, with the result being that a sharp image can be detected. Accordingly, in the FIA system, only by using white illumination light (having a wide band) and by adopting achromatism of a focusing optical system, the position of the wafer mark can be detected very accurately without being influenced by the photoresist layer.
In this regard, conventionally, since the photoresist layer is made of material having good permeability to light other than violet/ultraviolet light (exposure wavelength band), red/near-infrared light has been used mainly as the alignment illumination light so as not to sensitize the photoresist. Thus, for example, even when the width of the wavelength is selected to be 300 nm, a light band having a main wavelength of about 650 nm has been used, and a wavelength near the exposure wavelength has not been used to prevent the photoresist from being sensitized.
Also regarding the LIA systems, a technique has been proposed in which an influence of film interference of the photoresist layer is reduced by directing a plurality of pairs of laser beams having different wavelengths onto a diffraction grating wafer mark.
As mentioned above, in the conventional alignment systems, the influence of the interference of the photoresist layer is reduced by widening the bandwidth of the alignment illumination light or laser beam within the red/near-infrared wavelength range or by polychromatizing the illumination light.
However, recently, for example, when color liquid crystal panels or color CCDs are manufactured, in consideration of the fact that photoresist having low permeability to the red/near-infrared wavelength may be used, it has been required that a position of an alignment mark can be detected through a film having low permeability to the red/near-infrared wavelength.
That is to say, for example, in the manufacture of the color liquid crystal panels or color CCDs, red, green, blue or black photoresist (referred to as "color photoresist" hereinafter) materials are often used as the photoresist, and a color filter such as a red filter, a green filter or a blue filter is often used. In the case where such a color photoresist layer or color filter is used, when superimposing exposure is effected, the positions of the alignment marks provided under the color photoresist layer or color filter must be detected.
However, when the red/near-infrared light is used as the illumination light from the alignment system, since the red/near-infrared light is absorbed if the blue photoresist or blue filter is used, there arises a problem in which the position of the alignment mark cannot be detected. Accordingly, when the red/near-infrared light is used as the alignment light, as in the conventional case, depending on the colored thin films such as the color photoresist or the color filter to be used, the absorption of light occurs thereby making the detection of the wafer mark position impossible.